More than a million cases of non-melanoma skin cancers (i.e. basal cell and squamous cell carcinomas) are diagnosed every year. These cancers are more common than all other human cancers. Almost all basal cell and squamous cell carcinomas are curable by surgery if detected early. Unfortunately, in most cases the contrast of the lesions is poor, which complicates tumor localization and precise excision. Treatment of cancer patients could benefit greatly if a real-time non-invasive, reliable and cost-effective technique for delineating tumor margins were available. Therefore there is a strong need for practical, reliable, rapid, and precise methods for tumor localization, to guide surgery and other treatments of skin cancer. Once developed, such methods may potentially be useful for epithelial cancers of other organs. The overall goal of this proposal is to develop a practical way to detect skin cancer margins and guide tumor excision surgery using non-invasive optical imaging techniques. Specifically, spectrally resolved and polarization sensitive imaging methods and equipment, which are applicable for both reflectance and fluorescence imaging will be developed and implemented. The specific aims are: (i) Design and build multi-spectral polarization-enhanced reflectance and fluorescence imaging system for in vivo localization of skin cancers that can rapidly image large tumor affected areas in clinical setting. (ii) Develop methods for reliable localization of skin tumors based on polarized light reflectance imaging, dye enhanced polarized light reflectance imaging, and exogenous dye fluorescence-depolarization imaging. (iii) Develop and verify algorithms for quantitative assessment of skin tumor localization. (iv) Integrate the developed algorithms and imaging techniques into an efficient bedside guidance tool for tumor localization and excision. (v) Evaluate the clinical prototype and the developed imaging methods in a surgical practice environment.